Biomolecular Bombyx mori silk-based multiblock copolymers of poly(ethylene oxide)(PEO) and GlyAlaGlyAla prepared by step-growth polymerization based on a modular chemical method have been found to self-assemble into nanostructures. Solid-state C-13 NMR, solid-state FTIR, and powder X-ray diffraction provided evidence for the beta -sheet self-assembly. The overall beta -sheet content was as high as 90%. The building blocks had significant impact on the solid-state structure of the resulting polymer. Longer PEO spacers lowered the tendency to self-assemble into antiparallel beta -sheets and implicated the existence of PEG-sequestered isolated beta -sheets. Differential scanning calorimetry, transmission electron microscopy, and atomic force microscopy confirmed nanostructure formation and microphase separation. The microphase-separated morphology of the polymers contained 20-50 nm peptide domains dispersed in a continuous PEO phase on which was superimposed a 100-150 nm superstructure due to the polymers' polydispersity and multiblock character. Instron measurements suggested that mechanical properties could be modulated by manipulating the building blocks.